We studied the mechanisms by which T cells recognize antigens presented on the surface of other cells in association with major histocompatibility complex (MHC)-encoded molecules, and the application of these principles to the design of synthetic vaccines for AIDS, malaria, and cancer. We have been able to quantitate the requirements for class I MHC molecules and for antigenic peptide for stimulation of CD8+ cytotoxic T lymphocytes (CTL) using purified class I molecules on plastic. The titration curves for peptide and for MHC were interdependent; no accessory molecules were necessary; excess beta-2 microglobulin is necessary for peptide binding; conversely, peptide stabilizes association of beta-2 with MHC heavy chains. In studying HIV- specific CTL epitopes, we found that human CTL from HIV infected individuals recognized four peptides from the envelope in association with HLA-A2. One of these was a dominant CTL epitope in the mouse, but 3 had been identified as helper epitopes. Conversely, we found that the CTL epitope in the mouse also stimulated CD4+ helper T cells. To test the extent of this promiscuity of peptides for multiple MHC molecules, we studied three peptides in mice of 10 MHC types, and found that each was presented with several class I MHC molecules. Also class I-MHC presented CTL epitopes from HIV reverse transcriptase and the malaria circumsporozoite protein were also presented by class II MHC molecules to helper T cells. Use of this concordance between helper and CTL epitopes in vaccine design is being explored. To overcome the problem of MHC polymorphism in the human population, we showed that peptides spanning multideterminant regions of HIV gp160 were recognized by mice of multiple MHC types and by T cells from a large fraction of humans. We also found that IL-2 production in response to gp160 peptides may be an earlier diagnostic for infection or at least exposure to HIV than antibody responses. We have also begun to search for tumor infiltrating CTL specific for mutant oncogene products in human cancers and animal models, with the aim of developing peptide immunotherapy to induce such CTL.